1. Field of Invention
The present invention relates to an interface circuit which is used for fan speed monitoring and control, and more specifically to DC (direct current) fan speed monitoring and control.
2. Description of Related Art
Fan cooling is typically used to cool equipment, such as a computer or projector, when the power concentration of the equipment exceeds its natural cooling capability, and exhibits the significant advantage of superior heat dissipation as compared to natural cooling. Specifically, fan cooling has several times the heat removal capacity of natural cooling, and is less affected by the temperature of adjacent parts. A major disadvantage of fan cooling, however, is the contamination of internal parts by, for example, dust, oil fumes, and cigarette smoke carried into the equipment by the air stream. Although air filters can be used to reduce the effects of such contamination, failure to regularly clean the filter can result in slower fan speed, which consequently reduces the effectiveness of the fan cooling. The resulting higher working temperature of the equipment generally increases the failure rate of the system since a high Mean Time Between Failure (MTBF) requires low working temperatures. Furthermore, fan failure can result in overheating, and may cause permanent damage to the system.
Equipping a fan monitor to detect slow fan speed or fan failure provides information to warn the user of a dirty air-filter, or automatically locks the system into a low power mode when the fan fails. Such fan control conserves energy, which is advantageous to computer and other equipment manufacturers who have been striving to meet energy conservation requirements.
Many fan controllers and monitors have been proposed. Such fan monitors generally use either (1) a sensor positioned adjacent to the fan to sense the fan rotation; (2) a special fan with a tachometer output, which is generated by a Hall-effect switch; or (3) a current sensing resistor that senses the fan motor current and produces a tachometer pulse. These proposed fan controllers usually control fan speed by adjusting the fan voltage, in which the PWM (pulse width modulation) or the pulsed mode control prevents fan locking (freezing) at low speed or during the fan start-up period. In general, the input of the fan controller is an analog signal, which may be OR'ed with a thermal sensor signal for the fan speed control.
The following drawbacks, however, result from such prior art designs.
(1) It is inconvenient and expensive to install a rotation sensor or use a special fan instead of a standard fan. PA1 (2) When the fan rotates at high speed or low speed, the current flow through the fan is quite different. Consequently, it is difficult to design an adequate amplifier to amplify the signal that is sensed from the current sensing resistor. PA1 (3) The switching mode (PWM or pulse mode) fan control is too noisy for the amplifier to sense an accurate current and generate a correct tachometer pulse. Furthermore, the switching mode control produces not only an EMI (Electro-Magnetic Interference) problem, but also introduces a low frequency fan turning noise into the system through the coupled power supply. PA1 (4) Many microprocessors or micro-controllers include built-in counters for pulse counting, but most of the proposed fan monitors output a pass/fail signal only. The data acquired from these fan monitors is insufficient to analyze the characteristics of the fan. PA1 (5) In order to OR an input fan control signal with a thermal sensor signal, the input control signal of these proposed fan controllers is generally an analog signal. However, it is not economical to install a D/A converter specifically for the fan speed control. Moreover, for some low cost micro-controllers, it is easy to output a low frequency PWM signal (30.about.50 Hz) for the fan speed control. If the input of the fan speed controller can accept both analog and digital pulse (PWM) signals, this will help reduce the cost of the control circuit.
The disadvantages listed above reveal that prior art designs fail to meet significant needs of fan monitoring and control.